Differential device

ABSTRACT

A differential device includes: an input member inputted with driving force; a differential gear supported in the input member and capable of rotating relative to the input member and revolving around rotation center of the input member with rotation of the input member; paired output gears each including a tooth portion meshing with the differential gear and a shaft portion located radially inward of the tooth portion; a washer interposed between the input member and back surface of the tooth portion; and an oil groove provided in a recess shape in surface of the input member facing back surface of each output gear and extending from periphery of the shaft portion to back surface of the washer. The oil groove is arranged offset in peripheral direction of the output gear from a meshing portion between the tooth portion and the differential gear.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a differential device which is providedin a vehicle such as, for example, an automobile.

Description of the Related Art

Japanese Utility Model Registration No. 2606235 has made known aconventional differential device in which a washer is interposed betweena back surface of a tooth portion of each of output gears and an inputmember (for example, a differential case) and an oil groove guidinglubricant oil is provided in a recess shape in a surface of the inputmember facing the back surface of each output gear.

In the conventional device, on the surface of the input member facingthe back surface of each output gear, particularly on a region part ofsaid surface of the input member, which is located on a back surfaceside of a meshing portion between the output gear and a differentialgear, large thrust reaction force acts from the meshing portion via thetooth portion of the output gear and the washer.

In spite of the above situation, an oil groove in the conventionaldevice is formed in the region part located on the back surface side ofthe meshing portion in the surface of the input member facing the backsurface of the output gear, namely, the region part on which the largethrust reaction force acts. This may cause a decrease in supportrigidity of the region part which needs to bear large load burden, sothat durability of the region part, accordingly durability of the inputmember may deteriorate. In addition, due to concentration of the load onan edge portion of the oil groove, the durability of the input membermay deteriorate.

Furthermore, such problems may arise more seriously in a differentialdevice whose input member, in particular, is required to be reduced inthickness and weight, such as a differential device which is madethinner in an axial direction of the output gears of the differentialdevice by making the diameter of the output gears sufficiently largerthan that of the differential gears such that the output gears can havea sufficiently larger number of teeth than that of the differentialgears.

SUMMARY OF THE INVENTION

The present invention has been made with the foregoing situation takeninto consideration. An object of the present invention is to provide adifferential device capable of solving the above-mentioned problem witha simple structure.

In order to achieve the object, a differential device according to thepresent invention, comprises: an input member inputted with drivingforce; a differential gear supported in the input member and being ableto rotate relative to the input member and revolve around a rotationcenter of the input member in accordance with a rotation of the inputmember; a pair of output gears each including a tooth portion placed inmesh with the differential gear, and a shaft portion located radiallyinward of the tooth portion; a washer interposed between the inputmember and a back surface of the tooth portion of each of the outputgears; and an oil groove provided in a recess shape in a surface of theinput member facing a back surface of each of the output gears, the oilgroove extending from a periphery of the shaft portion of the outputgear to a back surface of the washer, wherein the oil groove is arrangedoffset in a peripheral direction of the output gear from a meshingportion between the tooth portion and the differential gear. (This is afirst characteristic of the present invention.)

According to the first characteristic, the differential device includes:the washer interposed between the input member and the back surface ofthe tooth portion of each of the output gears; and the oil groove whichis provided in the recess shape in the surface of the input memberfacing the back surface of the output gear, the oil groove extendingfrom the periphery of the shaft portion of the output gear to the backsurface of the washer. Thus, using centrifugal force, lubricant oil canbe effectively supplied from the periphery of the shaft portion of theoutput gear to the back surface of the washer via the oil groove.Accordingly, even when large thrust reaction force acts on the washerfrom the differential gear via the output gear, a sliding portionbetween the washer and the back surface of the output gear can besufficiently lubricated. In addition, the oil groove is arranged offsetin the peripheral direction of the output gear from the meshing portionbetween the tooth portion of the output gear and the differential gear.Thus, the oil groove can be placed out of particularly a region part, onwhich large thrust reaction force acts, in the surface of the inputmember facing the back surface of the output gear, namely, the regionpart located on the back surface side of the meshing portion. This caninhibit a decrease in support rigidity of the region part which needs tobear large load burden, and accordingly can contribute to an improvementof durability of the input member.

In the differential device according to the present invention,preferably, the input member includes a side wall portion facing theback surface of each of the output gears, the side wall portion includesa plurality of through-holes or recessed holes arranged at intervals ina peripheral direction of the input member, and the oil groove isarranged passing through between two of the through-holes or recessedholes placed adjacent to each other in the peripheral direction. (Thisis a second characteristic of the present invention.)

According to the second characteristic, the input member includes theside wall portion facing the back surface of each of the output gears;the side wall portion includes the plurality of through-holes orrecessed holes arranged at intervals in the peripheral direction of theinput member; and the oil groove is arranged passing through between twoof the through-holes or recessed holes placed adjacent to each other inthe peripheral direction. Thus, such provision of the through-holes orrecessed holes makes it possible to achieve a reduction in the weight ofthe input member while considering weight balance of the input member,and to form the sufficiently long oil groove while avoiding thethrough-holes or recessed holes (without the through-holes or the likeobstructing the oil groove).

In the differential device according to the present invention,preferably, an oil reserving portion is provided in a recess shape in aninner peripheral portion of a surface of the input member facing each ofthe output gears, the oil reserving portion facing an outer periphery ofthe shaft portion of the output gear. (This is a third characteristic ofthe present invention.)

According to the third characteristic, the oil reserving portion isprovided in the recess shape in the inner peripheral portion of thesurface of the input member facing each of the output gears, the oilreserving portion facing the outer periphery of the shaft portion of theoutput gear. Thus, supply of the lubricant oil to the oil groove can beappropriately adjusted using the oil reserving portion. For example, inan initial stage of a differential operation of the differential device,the lubricant oil can be smoothly supplied to the oil groove,accordingly to the washer and the like using the lubricant oil stored inthe oil reserving portion. On the other hand, an excess of the lubricantoil can be temporarily stored in the oil reserving portion to make thestored excess of the lubricant oil available when the supply of thelubricant oil to the oil groove becomes insufficient.

In the differential device according to the present invention,preferably, the oil groove is arranged near the meshing portion in theperipheral direction of the output gear. (This is a fourthcharacteristic of the present invention.)

According to the fourth characteristic, the oil groove is arranged nearthe meshing portion in the peripheral direction of the output gear. Thismakes it possible to place the oil groove as close as possible toparticularly the region part, which is acted on by the large thrustreaction force, in the surface of the input member facing the backsurface of each of the output gears, namely, the region part located onthe back surface side of the meshing portion, while placing the oilgroove out of the region part. Thereby, the region part which needs tobear large load burden can be effectively lubricated while inhibitingthe decrease in the support rigidity of the region part.

In the differential device according to the present invention,preferably, as seen in a projection plane orthogonal to a rotation axisof the output gears, a pair of the oil grooves are arranged with themeshing portions interposed therebetween. (This is a fifthcharacteristic of the present invention.)

According to the fifth characteristic, a pair of the oil grooves arearranged with the meshing portion interposed therebetween. The regionpart which needs to bear large load burden can be more effectivelylubricated while inhibiting the decrease in the support rigidity of theregion part.

In the differential device according to the present invention,preferably, the differential gear is supported in the input member via adifferential gear support portion supported in the input member, and

${d\; {2/{PCD}}} \leqq {3.36 \cdot \left( \frac{1}{Z\; 1} \right)^{\frac{2}{3}} \cdot {\sin \left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}$

is satisfied, and

Z1/Z2>2 is satisfied, where Z1, Z2, d2 and PCD denote the number ofteeth of each of the output gears, the number of teeth of thedifferential gear, a diameter of the differential gear support portionand a pitch cone distance, respectively. (This is a sixth characteristicof the present invention.)

According to the sixth characteristic, the differential device as awhole can be sufficiently reduced in width in the axial direction of theoutput shafts while securing the strength (for example, the statictorsion load strength) and the maximum amount of torque transmission atapproximately the same levels as the conventional differential device.Accordingly, the differential device can be easily incorporated in atransmission system, which is under many layout restrictions around thedifferential device, with great freedom and no specific difficulties,and is therefore advantageous in reducing the size of the transmissionsystem.

In the differential device according to the present invention,preferably, Z1/Z2≧4 is satisfied. (This is a seventh characteristic ofthe present invention.)

In the differential device according to the present invention,preferably, Z1/Z2≧5.8 is satisfied. (This is an eighth characteristic ofthe present invention.)

According to the seventh and eighth characteristics, the differentialdevice can be more sufficiently reduced in width in the axial directionof the output shafts while securing the strength (for example, thestatic torsion load strength) and the maximum amount of torquetransmission at approximately the same levels as the conventionaldifferential device.

The above and other objects, characteristics and advantages of thepresent invention will be clear from detailed descriptions of thepreferred embodiments which will be provided below while referring tothe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a main part in a differentialdevice and a speed reduction gear mechanism according to a firstembodiment of the present invention (a sectional view taken along a1A-1A line in FIG. 2).

FIG. 2 is a sectional view taken along a 2A-2A line in FIG. 1.

FIG. 3 is a sectional view taken along a 3A-3A line in FIG. 1.

FIG. 4 shows an enlarged view of a section indicated with an arrow 4A inFIG. 1, a partially enlarged view of the enlarged view of the sectionindicated with the arrow 4A in FIG. 1, and a load distribution chart.

FIG. 5 is an enlarged sectional view showing a main part (meshingportions between a pinion and side gears) of a differential deviceaccording to a second embodiment of the present invention.

FIG. 6 is an enlarged sectional view (a sectional view corresponding tothe partially enlarged view in FIG. 4) showing a main part of adifferential device according to a third embodiment of the presentinvention.

FIG. 7 is a longitudinal sectional view showing an example of aconventional differential device.

FIG. 8 is a graph showing a relationship of gear strength change rateswith a number-of-teeth ratio where the number of teeth of the pinion isset at 10.

FIG. 9 is a graph showing a relationship of the gear strength changerates with a pitch cone distance change rate.

FIG. 10 is a graph showing a relationship of the pitch cone distancechange rate with the number-of-teeth ratio for keeping 100% of the gearstrength where the number of teeth of the pinion is set at 10.

FIG. 11 is a graph showing a relationship between a shaft diameter/pitchcone distance ratio and the number-of-teeth ratio where the number ofteeth of the pinion is set at 10.

FIG. 12 is a graph showing a relationship between the shaftdiameter/pitch cone distance ratio and the number-of-teeth ratio wherethe number of teeth of the pinion is set at 6.

FIG. 13 is a graph showing a relationship between the shaftdiameter/pitch cone distance ratio and the number-of-teeth ratio wherethe number of teeth of the pinion is set at 12.

FIG. 14 is a graph showing a relationship between the shaftdiameter/pitch cone distance ratio and the number-of-teeth ratio wherethe number of teeth of the pinion is set at 20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below based onthe attached drawings.

First of all, referring to FIGS. 1 to 4, a first embodiment of thepresent invention will be described. In FIG. 1, a differential device Dis connected to an engine (not illustrated) as a power source mounted onan automobile via a speed reduction gear mechanism RG. The differentialdevice D drives a pair of axles arranged in parallel in a vehicle widthdirection while allowing differential rotation between the pair of axlesby distributively transmitting to output shafts J, J′ rotational forcewhich is transmitted from the engine to a differential case DC via thespeed reduction gear mechanism RG, the output shafts J, J′ beingcontinuous respectively to the pair of axles. The differential device Dis housed together with the speed reduction gear mechanism RG in, forexample, a transmission case M placed beside the engine in a frontportion of a vehicle body, in a way that the differential device D isadjacent to the speed reduction gear mechanism RG. Incidentally, a powerconnection-disconnection mechanism and a forward-rearward travelswitching mechanism (both not illustrated) which have been well-knownare installed between the engine and the speed reduction gear mechanismRG. In addition, a rotation axis L of the differential case DC coincideswith a center axis of the output shafts J, J′. Here, in the presentspecification, an “axial direction” means a direction along the centeraxis of the output shafts J, J′ (namely, a rotation axis L of thedifferential case DC and side gears S). In addition, a “radialdirection” means a radial direction of the differential case DC and theside gears S. Furthermore, a “back surface” means a surface on anopposite side in the axial direction of the side gears (output gears) Sfrom pinions (differential gears) P described later.

The speed reduction gear mechanism RG is formed, for example, from aplanetary gear mechanism including: a sun gear 20 which rotates inoperative connection with a crankshaft of the engine; a ring gear 21which concentrically surrounds the sun gear 20 and is fixed to an innerwall of the transmission case M; a plurality of planetary gears 22 whichare installed between the sun gear 20 and the ring gear 21 and mesh withthem; and a carrier 23 which rotatably and pivotally supports theplanetary gears 22. Incidentally, a speed reduction gear mechanismformed from a gear train including multiple spur gears may be usedinstead of such a planetary gear mechanism.

The carrier 23 is rotatably supported by the transmission case M via abearing not illustrated. Furthermore, in this embodiment, the carrier 23is joined to one end portion (a cover portion C′ described later) of thedifferential case DC of the differential device D so as to rotateintegrally with the differential case DC. Moreover, an other end portion(a cover portion C described later) of the differential case DC isrotatably supported in the transmission case M via a bearing 2. Thus, acombination body of the differential case DC and the carrier 23 whichintegrally rotate together is rotatably and stably supported in thetransmission case M via the plurality of bearings.

In addition, a through-hole Ma to be inserted with each of the outputshafts J, J′ is formed in the transmission case M. A seal member 3having an annular shape and sealing an interstice between an innerperiphery of the through-hole Ma and an outer periphery of each of theoutput shafts J, J′ is installed therebetween. Furthermore, an oil pan(not illustrated) which faces an inner space 1 of the transmission caseM and holds a predetermined amount of lubricant oil is provided in abottom portion of the transmission case M. In the inner space 1 of thetransmission case M, the lubricant oil held in the oil pan is scraped upand splashed by rotation of movable elements of the speed reduction gearmechanism RG, the differential case DC and the like toward vicinities ofrotational parts. This makes it possible to lubricate the mechanicalmoving parts existing inside and outside the differential case DC.

Incidentally, the lubricant oil held in the oil pan may be sucked in byan oil pump (not illustrated) to be forcibly injected or sprayed towardspecific parts in the inner space 1 of the transmission case M, forexample toward the speed reduction gear mechanism RG and thedifferential case DC, or toward an inner wall of the transmission case Min peripheries of the speed reduction gear mechanism RG and thedifferential case DC. In addition, an outer peripheral portion of thedifferential case DC of the embodiment may be either partially immersedor not immersed in the lubricant oil stored in an inner bottom portionof the transmission case M.

Referring to FIGS. 2 to 4 together, the differential device D includes:the differential case DC; a plurality of pinions P housed in thedifferential case DC; a pinion shaft PS housed in the differential caseDC and rotatably supporting the pinions P; and a pair of side gears Shoused in the differential case DC, meshing with the pinions Prespectively from both the left and right sides and connectedrespectively to the pair of output shafts J, J′. In this respect, eachside gear S is an example of an output gear; each pinion P is an exampleof a differential gear; and the differential case DC is an example of aninput member. Like in well-known conventional differential devices, theeach pinion P is housed and supported in the differential case DC, andis capable of rotating relative to the differential case DC andrevolving around a rotation center of the differential case DC inaccordance with a rotation of the differential case DC.

The differential case DC includes, for example: a case portion 4 havinga short cylindrical shape (a tubular shape) and supporting the pinionshaft PS such that the case portion 4 is capable of rotating with thepinion shaft PS; and a pair of cover portions C, C′ respectivelycovering outer sides of the pair of side gears S and rotating integrallywith the case portion 4.

One of the pair of cover portions C, C′, for example, the cover portionC′ which is on the speed reduction gear mechanism RG side, is formedseparately from the case portion 4 and detachably joined to the caseportion 4 using bolts B or other appropriate joining means. Moreover,the carrier 23 of the speed reduction gear mechanism RG is joined to thecover portion C′ using welding means or other appropriate joining meanssuch that the carrier 23 can rotate integrally with the cover portionC′. In addition, the cover portion C which is the other of the pair ofcover portions C, C′ is integrally formed in, for example, the caseportion 4 having the tubular shape. However, like the cover portion C′as one of the cover portions C, C′, the cover portion C as the other ofthe cover portions C, C′ may be formed separately from the case portion4 and joined to the case portion 4 using the bolts B or otherappropriate joining means.

Each of the cover portions C, C′ includes: a boss portion Cb whichconcentrically surrounds a shaft portion Sj described later of the sidegear S, in which the shaft portion Sj is rotatably fitted and supportedand being formed in a cylindrical shape; and a side wall portion Csformed in a plate shape and an annular shape and having an outer sidesurface all or most of which is a flat surface orthogonal to therotation axis L of the differential case DC, the side wall portion Csbeing integrally connected to an inner end in an axial direction of theboss portion Cb. An outer peripheral end of the side wall portion Cs isintegrally or detachably connected to the case portion 4. In addition,the side wall portions Cs of the cover portions C, C′ are arranged to besubstantially flush with or to protrude slightly from end surfaces inthe axial direction of the case portion 4. This inhibits the side wallportions Cs from protruding outward in the axial direction to a largeextent, and is accordingly advantageous in making the differentialdevice D flat in the axial direction.

Furthermore, in the side wall portion Cs of each of the cover portionsC, C′, a plurality of (for example, eight) through-holes H passingthrough so as to traverse the side wall portion Cs in the axialdirection are arranged side by side at intervals in a peripheraldirection. Places in which the through-holes H are formed, and the sizeof the through-holes H are appropriately set from a viewpoint ofsecuring weight balance and needed rigidity strength of each of thecover portions C, C′. However, instead of or in addition to thethrough-holes H like this, bottomed recessed holes opened only inwardmay be formed in an inner side surface of the side wall portion Cs ofeach of the cover portions C, C′. Incidentally, the employment of thethrough-holes H, in particular, makes it possible to guide the lubricantoil splashed inside the transmission case M into the differential caseDC via the through-holes H, and thus to more effectively lubricatesliding portions and meshing portions of the movable elements inside thedifferential case DC.

An outer peripheral surface of the output shaft J is relativelyrotatably fitted directly to an inner peripheral surface of the bossportion Cb of the one cover portion C. A recessed groove 8 is formed inthe inner peripheral surface of the boss portion Cb, the recessed groove8 having a spiral shape and being capable of forcedly feeding thelubricant oil from an outer end to an inner end in the axial directionof the boss portion Cb in accordance with relative rotation between theoutput shaft J and the boss portion Cb. Meanwhile, a recessed groove 8′is formed in an inner peripheral surface of the boss portion Cb of theother cover portion C′, the recessed groove 8′ having a spiral shape andbeing capable of forcedly feeding the lubricant oil from an outer end toan inner end in the axial direction of the boss portion Cb in accordancewith relative rotation between the boss portion Cb of the other coverportion C′ and the shaft portion Sj of the side gear S on the same sideas the other cover portion C′.

Then, the pinion shaft PS is arranged inside the differential case DC soas to be orthogonal to the rotation axis L of the differential case DC,and both end portions of the pinion shaft PS are respectively removablyinserted through a pair of support through-holes 4 a which are providedto the case portion 4 having the tubular shape, the supportthrough-holes 4 a being located on one diametric line of the caseportion 4. In addition, the pinion shaft PS is fixed to the case portion4 using a retaining pin 5 which penetrates through one end portion ofthe pinion shaft PS and is attached to the case portion 4. The retainingpin 5 is prevented from coming off the case portion 4 by abutting anouter end of the retaining pin 5 against the other cover portion C′.

Incidentally, the embodiment shows the differential device D whosepinion shaft PS is formed in a linear rod shape with the two pinions Prespectively supported by both end portions of the pinion shaft PS.Instead, the differential device D may include three or more pinions P.In this case, the pinion shaft PS is formed in a shape of crossing rodssuch that rods extend radially from the rotation axis L of thedifferential case DC in three or more directions corresponding to thethree or more pinions P (for example, in a shape of a cross when thedifferential device D includes four pinions P), and tip end portions ofthe pinion shaft PS support the pinions P, respectively. Furthermore,the case portion 4 is divided into a plurality of case elements suchthat the end portions of the pinion shaft PS can be attached to andsupported in the divided case elements.

Moreover, each pinion P may be directly fitted to the pinion shaft PS,or the pinion P may be fitted to the pinion shaft PS via bearing meanssuch as a bearing bush and the like. Incidentally, as shown in FIG. 2,the pinion shaft PS may be formed in a shape of a shaft whose diameteris substantially equal throughout its whole length, or formed in a shapeof a stepped shaft. Furthermore, in each fitting surface of the pinionshaft PS, which is fitted to the pinion P, a cutout surface 6 having aflat shape (see FIG. 2) is formed to secure a sufficient flow of thelubricant oil to the fitting surface. Thus, an oil passage through whichthe lubricant oil can flow is secured between the cutout surface 6 andthe inner peripheral surface of the pinion P.

Meanwhile, the pinions P and the side gears S are each formed as, forexample, a bevel gear. In addition, each pinion P as a whole and eachside gear S as a whole, including their tooth portions, are formed byplastic working such as forging and the like. For these reasons, theirtooth portions with an arbitrary gear ratio can be precisely formedwithout restriction in machining work in the case where the toothportions of the pinions P and the side gears S are formed by cuttingwork, and the like. Incidentally, as the pinions P and the side gears S,other types of gears may be used instead of the bevel gear. For example,a face gear may be used for the side gears S, while a spur gear or ahelical gear may be used for the pinions P.

In addition, the pair of side gears S each include: the shaft portion Sjto which an inner end portion of the corresponding one of the pair ofoutput shafts J, J′ is spline-fitted as at 7 and being formed in acylindrical shape; a tooth portion Sg formed in an annular shape andseparated radially outward from the shaft portion Sj, the tooth portionSg having a tooth surface in mesh with the pinions P; and anintermediate wall portion Sm formed in a flat ring plate shape extendingradially outward from the inner end portion of the shaft portion Sj toan inner peripheral end portion of the tooth portion Sg. Theintermediate wall portion Sm integrally connects the shaft portion Sjand the inner peripheral end portion of the tooth portion Sg. Inaddition, in a back surface f of each of the side gear S, a back surfaceportion fg of the tooth portion Sg protrudes outward in the axialdirection beyond a back surface portion fm of the intermediate wallportion Sm.

Incidentally, the shaft portions Sj of the side gears S are directly androtatably fitted in, for example, boss portions Cb of the cover portionsC, C′, respectively, but may be rotatably fitted in the boss portions Cbof the cover portions C, C′ via bearings, respectively.

In at least one of the left and right side gear S (in the embodiment, inboth the left and right side gears S), a plurality of penetrating oilpassages 9 are formed at intervals in a peripheral direction in theintermediate wall portion Sm, the penetrating oil passages 9 penetratingthrough the intermediate wall portion Sm so as to transverse theintermediate wall portion Sm in the axial direction. Thus, inside thedifferential case DC, the lubricant oil smoothly flows between an innerside and an outer side of the side gear S via the penetrating oilpassages 9. Incidentally, places in which the penetrating oil passages 9are formed, and the size of the penetrating oil passages 9 areappropriately set from a viewpoint of securing weight balance and neededrigidity strength of the side gear S.

In addition, the back surface portion fg of the tooth portion Sg of theside gear S (namely, a part of the back surface f of the side gears Swhich is located on the back surface side of a meshing portion I betweenthe side gear S and the pinion P) are rotatably abutted against andsupported on the inner side surface of the side wall portion Cs of eachof the cover portions C, C′, namely, a surface of the side wall portionCs facing the back surface f of the side gear S, via a washer W. Here,the washer W is fitted and retained in a washer retaining groove 10formed in at least one of the inner side surface of the side wallportion Cs of each of the cover portions C, C′ and a back surface of thetooth portion Sg of the corresponding side gear S (in this embodiment,the inner side surface of the side wall portion Cs).

Furthermore, an oil reserving portion T having an annular shape isformed in a recess shape in an inner peripheral end portion of the innerside surface of the side wall portion Cs of each of the cover portionsC, C′ (namely, the surface of the side wall portion Cs facing the backsurface f of the side gear S), the oil reserving portion T facing anouter periphery of the shaft portion Sj of the side gear S. In addition,particularly the oil reserving portion T in the cover portion Ccommunicates with an inner end in the axial direction of the recessedgroove 8 in the inner peripheral surface of the boss portion Cb via alubricant oil passage 11. The lubricant oil passage 11 is formed betweenfacing surfaces of an inner peripheral end portion of the boss portionCb of the cover portion C and an outer peripheral portion and an outerend surface of the shaft portion Sj of the side gear S on the coverportion C side. An outer end in the axial direction of the recessedgroove 8 is open to the inner space 1 of the transmission case M. Here,the inner end in the axial direction of the recessed groove 8 alsocommunicates with the spline-fitting portion 7 between an innerperipheral portion of the shaft portion Sj of the side gear S and anouter periphery of an inner end of the output shaft J. Thus, thespline-fitting portion 7 also can be supplied with the lubricant oil viathe recessed groove 8.

Meanwhile, the oil reserving portion T in the other cover portion C′communicates with an inner end in the axial direction of the recessedgroove 8′ formed in the inner peripheral surface of the boss portion Cbof the other cover portion C′. An outer end in the axial direction ofthe recessed groove 8′ communicates with the inner space 1 of thetransmission case M.

Furthermore, corresponding to that the back surface portion fg of thetooth portion Sg of the side gear S protrudes further outward in theaxial direction than the back surface portion fm of the intermediatewall portion Sm as described above, the inner side surface of the sidewall portion Cs of each of the cover portions C, C′ are formed such thata part of the side wall portion Cs corresponding to the back surfaceportion fm of the intermediate wall portion Sm protrudes further inwardin the axial direction (is thicker in the axial direction) than a partof the side wall portion Cs corresponding to the back surface portion fgof the tooth portion Sg. This makes it possible to form the intermediatewall portions Sm of the side gears S as thin as possible whilesufficiently securing support rigidity of the cover portions C, C′(accordingly, the differential case DC) with respect to the backsurfaces of the tooth portions Sg of the side gears S, and therefore itis possible to achieve a further reduction in the weight of thedifferential device D and a further reduction in the thickness of thedifferential device D in the axial direction.

Moreover, in each of the cover portions C, C′, a plurality of oilgrooves G are formed each in a recess shape in the inner side surface ofthe side wall portion Cs (namely, the surface of the side wall portionCs facing the back surface f of the corresponding side gear S), the oilgrooves G extending linearly from a periphery of the shaft portion Sj ofthe side gear S to the back surface of the washer W. The plurality ofoil grooves G are arranged offset in a peripheral direction of the sidegear S from the meshing portions I between the tooth portion Sg of theside gear S and the pinions P, as shown in FIG. 3 in particular.

Particularly, the oil grooves G of the embodiment are arranged extendingradially with respect to the rotation axis L of the differential case DCand each passing through between two through-holes H which are adjacentto each other in the peripheral direction of the side gear S. That is,as seen in a projection plane orthogonal to the rotation axis L of theside gear S, the oil grooves G are each arranged in a position notoverlapping the pinions P in the peripheral direction. Furthermore, asseen in the projection plane orthogonal to the rotation axis L of theside gear S (FIG. 3), each pair of the oil grooves G are arranged in a Vshape with the corresponding meshing portion I between the side gear Sand the pinion P interposed between each pair of the oil grooves G, andalso arranged near the meshing portion I. Moreover, an inner end in theradial direction of each oil groove G directly communicates with the oilreserving portion T. Incidentally, for example, each pair of the oilgrooves G arranged with the meshing portion I interposed therebetweenmay be arranged in parallel to each other along the pinion shaft PS,instead of in the V shape as in the embodiment.

Meanwhile, as shown in FIG. 4, in the back surface f of each side gearS, an outermost peripheral end fwe of a washer abutting surface fw whichabuts against the washer W is located in the same position in the radialdirection of the side gear S as an outermost peripheral end Ie of themeshing portion I between the side gear S and the pinion P, and an outerperipheral end portion We of the washer W extends further radiallyoutward than the washer abutting surface fw. Besides, in the embodiment,in each side gear S, the outermost peripheral end fwe of the washerabutting surface fw of the side gear S is a largest outer diameterportion of the side gear S.

Next, descriptions will be provided for an operation of the firstembodiment. In the differential device D of this embodiment, in a casewhere the differential case DC receives rotational force from the enginevia the speed reduction gear mechanism RG, when the pinion P revolvesaround the rotation axis L of the differential case DC together with thedifferential case DC, without rotating around the pinion shaft PS, theleft and right side gears S are rotationally driven at the same speedfrom the differential case DC via the pinions P, and driving forces ofthe side gears S are evenly transmitted to the left and right outputshafts J, J′. Meanwhile, when a difference in rotational speed occursbetween the left and right output shafts J, J′ due to turn traveling orthe like of the automobile, the pinion P revolves around the rotationaxis L of the differential case DC while rotating around the pinionshaft PS. Thereby, the rotational driving force is transmitted from thepinion P to the left and right side gears S while allowing differentialrotations. The above is the same as the operation of the conventionaldifferential device.

Meanwhile, in a case where the power of the engine is being transmittedto the left and right output shafts J, J′ via the speed reduction gearmechanism RG and the differential device D while the automobile is, forexample, travelling forward, the lubricant oil is powerfully splashed invarious areas inside the transmission case M due to the rotation of themovable elements of the speed reduction gear mechanism RG and therotation of the differential case DC. As described above, part of thesplashed lubricant oil flows into the differential case DC from theplurality of through holes H. Thereafter, by centrifugal force, part ofthe inflowing lubricant oil flows along gaps between the side wallportions Cs of the cover portions C, C′ and the back surfaces f of theside gears S toward sliding portions between the tooth portions Sg ofthe side gears S and the washers W, and then lubricates the slidingportions. Meanwhile, other part of the lubricant oil having flowed intothe differential case DC also flows into spaces inside the side gears Svia the penetrating oil passages 9 in the side gears S. Subsequently, bythe centrifugal force, the inflowing part of the lubricant oil flowsradially outward along inner side surfaces of the side gears S, andreaches the tooth surfaces of the tooth portions Sg of the side gears Sand the meshing portions I between the tooth portions Sg of the sidegears S and the pinions P, and then lubricates the meshing portions I.

Moreover, in accordance with the relative rotation between the bossportion Cb of the one cover portion C of the differential case DC andthe output shaft J, part of the lubricant oil reaching a vicinity of anouter end of the boss portion Cb after splashed into the transmissioncase M is fed toward the inner end side in the axial direction of theboss portion Cb via the recessed groove 8 in the inner peripheralsurface of the boss portion Cb, and flows from the inner end in theaxial direction of the recessed groove 8 into the inner ends in theradial direction of the oil grooves G after sequentially passing throughthe lubricant oil passage 11 and the oil reserving portion T.Incidentally, part of the lubricant oil reaching the inner end in theaxial direction of the recessed groove 8 also flows into thespline-fitting portion 7, and thereafter flows from the spline-fittingportion 7 into the inner side surface side of the side gear S.

On the other hand, in accordance with the relative rotation between theboss portion Cb of the other cover portion C′ of the differential caseDC and the shaft portion Sj of the corresponding side gear S, part ofthe lubricant oil reaching a vicinity of an outer end of the bossportion Cb after splashed into the transmission case M is fed toward theinner end in the axial direction of the boss portion Cb via the recessedgroove 8′ in the inner peripheral surface of the boss portion Cb, andflows from the inner end in the axial direction of the recessed groove8′ into the inner ends in the radial direction of the oil grooves G viathe oil reserving portion T.

According to this embodiment, each side gear S includes the intermediatewall portion Sm having a flat ring plate shape and connecting betweenthe shaft portion Sj on an inner peripheral side of the side gear S andthe tooth portion Sg on an outer peripheral side of the side gear S, thetooth portion Sg being separated outward from the shaft portion Sj in aradial direction of the side gear S. The width t1 in the radialdirection of the intermediate wall portion Sm is larger than the maximumdiameter d1 of each pinion P. For these reasons, the diameter of eachside gear S can be made sufficiently larger than the diameter of thepinion P, so that the number Z1 of teeth of the side gear S can be madesufficiently larger than the number Z2 of teeth of the pinion P, and itis possible to reduce load burden on the pinion shaft PS in torquetransmission from the pinions P to the side gears S. Thus, it ispossible to decrease the effective diameter d2 of the pinion shaft PS,and accordingly to decrease a width (diameter) of each pinion P in theaxial direction of the output shafts J, J′.

Furthermore, in this manner, load burden on the pinion shaft PS isreduced, and reaction force applied to the side gears S decreases. Inaddition, the back surfaces f of the side gears S (particularly, theback surface portions fg located on the back surface side of the meshingportions I between the side gears S and the pinions P) are supported onthe side wall portions Cs of the cover portions C, C′ via the washers W.Therefore, it is easy to secure the rigidity strength needed for each ofthe side gears S even if the intermediate wall portion Sm is thinned.That is, it is possible to sufficiently thin the intermediate wallportion Sm of the side gear S while securing the support rigidity withrespect to the side gear S. Moreover, in the embodiment, since themaximum thickness t2 of the intermediate wall portion Sm of the sidegear S is formed much smaller than the effective diameter d2 of thepinion shaft PS whose diameter can be made smaller, the further thinningof the intermediate wall portion Sm of the side gear S can be achieved.Besides, since the side wall portion Cs of each of the cover portions C,C′ is formed in a flat plate shape such that the outer side surface ofthe side wall portion Cs is the flat surface orthogonal to the rotationaxis L of the differential case DC, the thinning of the side wallportion Cs itself of each of the cover portions C, C′ can be achieved.

Moreover, according to the embodiment, in the back surface f of the sidegear S, the back surface portion fg of the tooth portion Sg protrudesfurther outward in the axial direction than the back surface portion fmof the intermediate wall portion Sm. This makes it possible to form theintermediate wall portion Sm of the side gear S as thin as possiblewhile sufficiently securing rigidity of the tooth portion Sg of the sidegear S, accordingly it is possible to achieve the reduction in theweight of the differential device D and the reduction in the thicknessof the differential device D in the axial direction.

As a result of these, the width of the differential device D as a wholecan be sufficiently decreased in the axial direction of the outputshafts J, J′ while securing as approximately the same strength (forexample, static torsion load strength) and as approximately the sameamount of maximum torque transmission compared with the conventionaldifferential device. This makes it possible to easily incorporate thedifferential device D in a transmission system, which is under manylayout restrictions around the differential device D, with great freedomand no specific difficulties, and is extremely advantageous in reducingthe size of the transmission system of the differential device D.

In addition, according to the embodiment, due to the centrifugal force,most of the lubricant oil flowing into the oil grooves G in the coverportions C, C′ smoothly flows radially outward in the oil grooves G, andis efficiently supplied to the back surfaces of the washers W. Thus,even when large thrust reaction force acts on the washers W from thepinions P via the side gears S, it is possible to sufficiently lubricatesliding portions between the washers W and the back surfaces f of theside gears S (particularly, the back surface portions fg of the toothportions Sg). Furthermore, each of the oil grooves G are arranged offsetin the peripheral direction of the side gear S from the meshing portionI between the tooth portion Sg of the side gear S and the pinion P.Accordingly, the oil groove G can be placed in the peripheral directionout of particularly a region part acted on by large thrust reactionforce in a surface of the differential case DC (namely, the side wallportions Cs of each of the cover portions C, C′) facing the back surfacef of the corresponding side gear S, namely the region part located onthe back surface side of the meshing portion I. This inhibits a decreasein support rigidity of the region part where large load burden isapplied to the differential case DC. Accordingly, an improvement in thedurability of the differential case DC can be achieved.

Furthermore, according to the embodiment, the plurality of through-holesH are arranged side by side at intervals in the peripheral direction inthe side wall portion Cs of each of the cover portions C, C′ in thedifferential case DC, and each of the oil grooves G passes throughbetween two through-holes H which are adjacent to each other. Suchprovision of the through-holes H advantageously makes it possible notonly to achieve the reduction in the weight of the differential case DCwhile considering the weight balance of the differential case DC, butalso to form the sufficiently long oil grooves G while avoiding thethrough-holes H (that is, without the through-holes H or the likeobstructing the oil grooves G).

Moreover, according to the embodiment, as seen in the projection plane(FIG. 3) orthogonal to the rotation axis L of the side gear S, thewasher W and the back surface portion fg, which is a part of the backsurface f of the side gear S and located on the back surface side of themeshing portion I, are arranged partially overlapping each other. Thus,in the surfaces of the differential case DC (namely, the inner sidesurfaces of the side wall portions Cs of the side covers C, C′) facingthe back surfaces f of the side gears S, particularly to the regionparts acted on by the large thrust reaction force in the surfaces of thedifferential case DC, the thrust reaction force is transmitted from theside gears S via the washers W. This makes it possible to avoid loadconcentration on the region parts. Thereby, it is possible to moreeffectively inhibit the decrease in the support rigidity of the regionparts which need to bear large load burden. Accordingly, a furtherimprovement in the durability of the differential case DC can beachieved.

Besides, according to the embodiment, the oil reserving portions T arerespectively formed each in the recess shape in the inner peripheral endportions of the surfaces of the differential case DC facing the sidegears S (namely, the inner peripheral end portions of the inner sidesurfaces of the side wall portions Cs of the cover portions C, C′), theoil reserving portions T facing the outer peripheries of the shaftportions Sj of the side gears S. This makes it possible to appropriatelyadjust a supply amount of the lubricant oil to the oil grooves G byusing the oil reserving portions T. For example, in an initial stage ofa differential operation of the differential device D, the lubricant oilcan be smoothly supplied to the oil grooves G, accordingly to thewashers W and the back surfaces f of the side gears S by using thelubricant oil stored in the oil reserving portions T. On the other hand,an excess of the lubricant oil can be temporarily stored in the oilreserving portions T to make the stored excess of the lubricant oilavailable when the supply of the lubricant oil to the oil grooves Gbecomes insufficient.

Moreover, according to the embodiment, the oil grooves G are arrangednear the meshing portions I in the peripheral direction of the sidegears S. This makes it possible to place the oil grooves G as close aspossible to particularly the region parts acted on by the large thrustreaction force in the surfaces of the differential case DC facing theback surfaces f of the side gears S, namely, the region parts located onthe back surface side of the meshing portions I, while placing the oilgrooves G out of the region parts. Consequently, the region parts in thedifferential case DC which need to bear large load burden can beeffectively lubricated while inhibiting the decrease in the supportrigidity of the region parts as much as possible. Furthermore, since theoil grooves G like this are arranged in pair with the meshing portion Iinterposed each pair of the oil grooves G, the region parts which needto bear large load burden can be more effectively lubricated whileinhibiting the decrease in the support rigidity of the region parts.

Further, according to this embodiment, even in a case where the toothportions Sg of the side gears S place farther from the output shafts J,J′ due to increase in the diameter of the side gears S, or even undersevere driving conditions such as the high-speed rotation of the pinionsP, the lubricant oil can be efficiently supplied to the meshing portionsI and the sliding portions between the back surfaces f of the side gearsS and the washers W. Accordingly, the seizure in the meshing portions Iand the sliding portions can be prevented effectively.

Meanwhile, in the embodiment, as shown in FIG. 4, in the back surface fof each side gear S, the outermost peripheral end fwe of the washerabutting surface fw, which abuts against the washer W, in the sameposition in the radial direction of the side gear S as the outermostperipheral end Ie of the meshing portion I between the side gear S andthe pinion P. Thus, large thrust reaction force from the pinion P viathe tooth portion Sg in the outer periphery of the side gear S is lesslikely to concentrate excessively on an outermost peripheral end portionof the washer abutting surface fw of the side gear S, and the loadburden on the tooth portion Sg itself in the outer periphery of the sidegear S also decreases. Incidentally, in the present invention, thewasher abutting surface fw may be set such that the outermost peripheralend fwe of the washer abutting surface fw is located outward in theradial direction of the side gear S with respect to the outermostperipheral ends Ie of the meshing portions I. Also in this case, thesame effect as described above can be expected.

Moreover, the outer peripheral end portion We of the washer W extendsfurther radially outward than the washer abutting surface fw of the sidegear S. Thus, as being clear from a load distribution chart in FIG. 4,load distribution is performed on a washer receiving portion of thedifferential case DC (namely, a bottom portion of the washer retaininggroove 10 in the side wall portion Cs of each of the cover portions C,C′). Thereby, it is possible to effectively avoid a local increase inthe load burden on the washer receiving portion. Incidentally, in theload distribution chart in FIG. 4, a comparative example (indicated witha dashed line) represents a case where the outer peripheral end portionWe of the washer W is not extended further radially outward than thewasher abutting surface fw of the side gear S. In the comparativeexample, the load burden is too large on the washer receiving portion ofthe differential case DC which is in contact with an outermostperipheral end of the washer W.

The above-described configuration of the embodiment having therelationship among the back surfaces f of the side gears S, the washersW, and the washer receiving portions of the differential case DC canachieve reductions in the thicknesses and weights of the differentialcase DC (particularly, the side wall portions Cs of the cover portionsC, C′) and the side gears S (particularly, the tooth portions Sg in theouter peripheries of the side gears S), and can contribute to thereduction in the thickness of the differential device D in the axialdirection, and the reduction in the weight of the differential device D.Moreover, since the outermost peripheral ends fwe of the washer abuttingsurfaces fw are the largest outer diameter portions of the side gears S,the large thrust reaction force can be appropriately distributed on andreceived by the washer receiving portions of the differential case DCwithout unnecessarily increasing the diameters of the side gears S. Thismakes it possible to achieve further decreases in the thicknesses andweights of the side wall portions Cs of the differential case DC and thetooth portions Sg of the side gears S.

Next, using FIG. 5, descriptions will be provided for a secondembodiment of the present invention. Incidentally, constituentcomponents which are the same as those of the first embodiment will bedenoted by the same reference signs, and detailed descriptions for suchconstituent components will be omitted.

Although the first embodiment has shown the differential device whichuses the long pinion shaft PS as the support portion supporting thepinions P (that is, a differential gear support portion), this secondembodiment shows a differential device which is configured such that thesupport portion supporting the pinions P (that is, the differential gearsupport portion) is formed from a support shaft PS' coaxially andintegrally connected to a large diameter-side end surface of the pinionP. According to this configuration, it is unnecessary to provide in thepinion P the through-hole fitted with the pinion shaft PS, and thus itis possible to reduce the diameter (the width in the axial direction) ofthe pinion P by an amount corresponding to the through-hole. Thereby,the differential device D can be further thinned in the axial directionof the output shafts J, J′. In other words, in a case where the pinionshaft PS penetrates through the pinion P, it is necessary to form in thepinion P the through-hole in a size corresponding to the diameter of thepinion shaft PS. In contrast, in a case where the support shaft PS'integrated with the end surface of the pinion P, it is possible toreduce the diameter of the pinion P (the width of the pinion P in theaxial direction of the output shafts J, J′) without depending on anouter diameter (that is, the effective diameter d2) of the support shaftPS′.

Furthermore, as bearing means, a bearing bush 12 is installed between anouter peripheral surface of the support shaft PS' and an innerperipheral surface of a corresponding support through-hole 4 a providedto the outer peripheral wall, that is, the case portion 4 having thetubular shape, of the differential case DC. The bearing bush 12 isconfigured to allow relative rotation between the outer peripheralsurface of the support shaft PS' and the inner peripheral surface of thesupport through-hole 4 a. Incidentally, a bearing such as a needlebearing and the like may be used as the bearing means. Otherwise, thebearing may be omitted so that the support shaft PS' is directly fittedin the support through-hole 4 a of the differential case DC.

The second embodiment of the present invention except for the abovedescriptions can achieve the substantially same effect as that of thefirst embodiment.

Next, using FIG. 6, descriptions will be provided for a third embodimentof the present invention. In the first and second embodiments, theoutermost peripheral end fwe of the washer abutting surface fw, whichabuts against the washer W, of the back surface f of the side gear S islocated in the same position as or further outward in the radialdirection of the side gear S than, the outermost peripheral ends Ie ofthe meshing portion I between the side gear S and the pinion P, and theoutermost peripheral end fwe of the washer abutting surface fw is thelargest outer diameter portion of the side gear S. However, in the thirdembodiment, a rounded portion r having an arc shape in a cross sectionsmoothly connects between an outer peripheral end surface of the toothportion Sg of the side gear S and the back surface of the tooth portionSg (particularly, the washer abutting surface fw) of the side gear S.Thus, the outermost peripheral end fwe of the washer abutting surface fwis located further radially inward than the largest outer diameterportion of the side gear S (namely, the outer peripheral end surface).Like in the first and second embodiments, however, the outer peripheralend portion We of the washer W extends further radially outward than thewasher abutting surface fw, and the washer abutting surface fw islocated on the back surface side of the meshing portion I.

The other configuration of the third embodiment is the same as that ofthe first embodiment, and therefore the constituent components will bedenoted by the same reference sings of the corresponding constituentcomponents of the first embodiment, and duplicated descriptions for suchconstituent components will be omitted.

Accordingly, also in the third embodiment, it is possible to achieve thesubstantially same effect as the first and second embodiments.Incidentally, in the third embodiment, in the side gear S, the outerperipheral end surface of the tooth portion Sg and the back surface ofthe tooth portion Sg (particularly, the washer abutting surface fw) maybe connected together by a flat taper surface having a straight lineshape in a cross section, instead of the rounded portion r.

Meanwhile, in the conventional differential devices (particularly, theconventional differential devices each including a pinion (differentialgear) inside an input member, and a pair of side gears (output gears)meshing with the pinion (differential gear)) exemplified in JapanesePatent No. 4803871 and Japanese Patent Application KOKAI Publication No.2002-364728, the number Z1 of teeth of the side gear (output gear) andthe number Z2 of teeth of the pinion (differential gear) are generallyset at 14 and 10, 16 and 10, or 13 and 9, respectively, as shown inJapanese Patent Application KOKAI Publication No. 2002-364728, forexample. In these cases, the number-of-teeth ratios Z1/Z2 of the outputgears to the differential gears are 1.4, 1.6 and 1.44, respectively. Inaddition, other publicly-known examples of the combination of the numberZ1 of teeth and the number Z2 of teeth for conventional differentialdevices include 15 and 10, 17 and 10, 18 and 10, 19 and 10, and 20 and10. In these cases, the number-of-teeth ratios Z1/Z2 are at 1.5, 1.7,1.8, 1.9 and 2.0, respectively.

On the other hand, nowadays, there is an increase in the number oftransmission systems which are under layout restrictions around theirrespective differential devices. Accordingly, the market demands thatdifferential devices be sufficiently reduced in width (i.e., thinned) inthe axial direction of their output shafts while securing the gearstrength for the differential devices. However, the structural forms ofthe conventional existing differential devices are wide in the axialdirection of the output shafts, as apparent from the gear combinationsleading to the above-mentioned number-of-teeth ratios. This makes itdifficult to satisfy the market demand.

With this taken into consideration, an attempt to find a concreteconfiguration example of the differential device D which can besufficiently reduced in width (i.e., thinned) in the axial direction ofthe output shafts while securing the gear strength for the differentialdevice has been made as follows, from a viewpoint different from that ofthe foregoing embodiments. Incidentally, the structures of thecomponents of the differential device D of this configuration exampleare the same as the structures of the components of the differentialdevice D of the foregoing embodiments which have been described usingFIGS. 1 to 6 (particularly, FIGS. 1 to 4 and FIG. 6). For this reason,the components of the configuration example will be denoted with thesame reference signs as those of the embodiments, and descriptions forthe structures will be omitted.

To begin with, let us explain a basic concept for sufficiently reducingthe width of (i.e., thinning) the differential device D in the axialdirection of the output shafts J, J′ referring to FIG. 7 together. Theconcept is as follows.

Approach [1] To make the number-of-teeth ratio Z1/Z2 of the side gear S,that is, the output gear to the pinion P, that is, the differential gearlarger than the number-of-teeth ratio used for the conventional existingdifferential device. (This leads to a decrease in the module(accordingly the tooth thickness) of the gear and a resultant decreasein the gear strength, while leading to an increase in the pitch circlediameter of the side gear S, a resultant decrease in transmission loadin the meshing portion of the gear, and a resultant increase in the gearstrength. However, the gear strength as a whole decreases, as discussedbelow.)Approach [2] To make the pitch cone distance PCD of the pinion P largerthan the pitch cone distance in the conventional existing differentialdevice. (This leads to an increase in the module of the gear and aresultant increase in the gear strength, while leading to an increase inthe pitch circle diameter of the side gear S, a resultant decrease inthe transmission load in the meshing portion of the gear, and aresultant increase in the gear strength. Thus, the gear strength as awhole increases greatly, as discussed below.)

For these reasons, when the number-of-teeth ratio Z1/Z2 and the pitchcone distance PCD are set such that the amount of decrease in the gearstrength based on Approach [1] is equal to the amount of increase in thegear strength based on Approach [2] or such that the amount of increasein the gear strength based on Approach [2] is greater than the amount ofdecrease in the gear strength based on Approach [1], the gear strengthas a whole can be made equal to or greater than that of the conventionalexisting differential device.

Next, let us concretely examine how the gear strength changes based onApproaches [1] and [2] using mathematical expressions. Incidentally, theexamination will be described in the following embodiment. First of all,a “reference differential device” is defined as a differential device D′in which the number Z1 of teeth of the side gear S is set at 14 whilethe number Z2 of teeth of the pinion P is set at 10. In addition, foreach variable, a “change rate” is defined as a rate of change in thevariable in comparison with the corresponding base number (i.e., 100%)of the reference differential device D′.

Approach [1]

When MO, PD₁, θ₁, PCD, FO, and TO respectively denote the module, pitchcircle diameter, pitch angle, pitch cone distance, transmission load inthe gear meshing portion, and transmission torque in the gear meshingportion, of the side gear S, general formulae concerning the bevel gearprovide

MO=PD ₁ /Z1,

PD ₁=2PCD·sin θ₁, and

θ₁=tan⁻¹(Z1/Z2).

From these expressions, the module of the gear is expressed with

MO=2PCD·sin {tan⁻¹(Z1/Z2)}/Z1.  (1)

Meanwhile, the module of the reference differential device D′ isexpressed with

2PCD·sin {tan⁻¹(7/5)}/14.

Dividing the term on the right side of Expression (1) by 2PCD·sin{tan⁻¹(7/5)}/14 yields a module change rate with respect to thereference differential device D′, which is expressed with Expression (2)given below.

$\begin{matrix}{{{Module}\mspace{14mu} {Change}\mspace{14mu} {Rate}} = \frac{14 \cdot {\sin \left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}{Z\; {1 \cdot {\sin \left( {\tan^{- 1}\frac{7}{5}} \right)}}}} & (2)\end{matrix}$

In addition, the section modulus of the tooth portion corresponding tothe gear strength (i.e., the bending strength of the tooth portion) isin proportion to the square of the tooth thickness, while the tooththickness has a substantially linear relationship with the module MO.For these reasons, the square of the module change rate corresponds to arate of change in the section modulus of the tooth portion, accordinglya gear strength change rate. In other words, based on Expression (2)given above, the gear strength change rate is expressed with Expression(3) given below. Expression (3) is represented by a line L1 in FIG. 8when the number Z2 of teeth of the pinion P is 10. From the line L1, itis learned that as the number-of-teeth ratio Z1/Z2 becomes larger, themodule becomes smaller and the gear strength accordingly becomes lower.

$\begin{matrix}{\begin{matrix}{{{Gear}\mspace{14mu} {Strength}\mspace{14mu} {Change}\mspace{14mu} {Rate}} = \left( {{Module}\mspace{14mu} {Change}\mspace{14mu} {Rate}} \right)^{2}} \\{= \frac{196 \cdot {\sin^{2}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}{Z\; {1^{2} \cdot {\sin^{2}\left( {\tan^{- 1}\frac{7}{5}} \right)}}}}\end{matrix}\quad} & (3)\end{matrix}$

Meanwhile, based on the general formulae concerning the bevel gear, atorque transmission distance of the side gear S is expressed withExpression (4) given below.

PD ₁/2=PCD·sin {tan⁻¹(Z1/Z2)}  (4)

From the torque transmission distance PD₁/2, the transmission load FO isgiven as

FO=2TO/PD ₁.

For this reason, when the torque TO of the side gear S of the referencedifferential device D′ is constant, the transmission load FO is ininverse proportion to the pitch circle diameter PD₁. In addition, therate of change in the transmission load FO is in inverse proportion tothe gear strength change rate. For this reason, the gear strength changerate is equal to the rate of change in the pitch circle diameter PD₁.

As a result, using Expression (4), the rate of change in the pitchcircle diameter PD₁ is expressed with Expression (5) given below.

$\begin{matrix}{\begin{matrix}{{{Gear}\mspace{14mu} {Strength}\mspace{14mu} {Change}\mspace{14mu} {Rate}} = {{PD}_{1}\mspace{14mu} {Change}\mspace{14mu} {Rate}}} \\{= {\frac{\sin \left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}{\sin \left( {\tan^{- 1}\frac{7}{5}} \right)}\quad}}\end{matrix}\quad} & (5)\end{matrix}$

Expression (5) is represented by a line L2 in FIG. 8 when the number Z2of teeth of the pinion P is 10. From the line L2, it is learned that asthe number-of-teeth ratio Z1/Z2 becomes larger, the transmission loadbecomes smaller, and the gear strength accordingly becomes stronger.

Eventually, the gear strength change rate in accordance with theincrease in the number-of-teeth ratio Z1/Z2 is expressed with Expression(6) given below by multiplying a rate of decrease change in the gearstrength in accordance with the decrease in the module MO (the term onthe right side of Expression (3) shown above) and a rate of increasechange in the gear strength in accordance with the decrease in thetransmission load (the term on the right side of Expression (5) shownabove).

$\begin{matrix}{{{Gear}\mspace{14mu} {Strength}\mspace{14mu} {Change}\mspace{14mu} {Rate}\mspace{14mu} {in}\mspace{14mu} {Accordance}\mspace{14mu} {with}\mspace{14mu} {Number}\text{-}{of}\text{-}{Teeth}\mspace{14mu} {Ratio}} = \frac{196 \cdot {\sin^{3}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}{Z\; {1^{2} \cdot {\sin^{3}\left( {\tan^{- 1}\frac{7}{5}} \right)}}}} & (6)\end{matrix}$

Expression (6) is represented by a line L3 in FIG. 8 when the number Z2of teeth of the pinion P is 10. From the line L3, it is learned that asthe number-of-teeth ratio Z1/Z2 becomes larger, the gear strength as awhole becomes lower.

Approach [2]

In a case of increasing the pitch cone distance PCD of the pinion P morethan the pitch cone distance in the reference differential device D′,when PCD1, PCD2 respectively denote the pitch cone distance PCD beforethe change and the pitch cone distance PCD after the change, the modulechange rate in accordance with the change in the pitch cone distance PCDis expressed with

PCD2/PCD1

if the number of teeth is constant, based on the above-mentioned generalformulae concerning the bevel gear.

Meanwhile, as being clear from the above-discussed process for derivingExpression (3), the gear strength change rate of the side gear Scorresponds to the square of the module change rate. For this reason,

Gear Strength Change Rage in Accordance with Increase inModule=(PCD2/PCD1)²  (7)

is obtained. Expression (7) is represented by a line L4 in FIG. 9. Fromthe line L4, it is learned that as the pitch cone distance PCD becomeslarger, the module becomes larger, and the gear strength accordinglybecomes stronger.

In addition, when the pitch cone distance PCD is made larger than thepitch cone distance PCD1 in the reference differential device D′, thetransmission load FO decreases. Thereby, the gear strength change ratebecomes equal to the rate of change in the pitch circle diameter PD₁, asdescribed above. In addition, the pitch circle diameter PD₁ of the sidegear S is in proportion to the pitch cone distance PCD. For thesereasons,

Gear Strength Change Rate in Accordance with Decrease in TransmissionLoad=PCD2/PCD1  (8)

is obtained.

Expression (8) is represented by a line L5 in FIG. 9. From the line L5,it is learned that as the pitch cone distance PCD becomes larger, thetransmission load becomes lower, and the gear strength accordinglybecomes stronger.

In addition, the gear strength change rate in accordance with theincrease in the pitch cone distance PCD is expressed with Expression (9)given below by multiplying the rate of increase change in the gearstrength in accordance with the increase in the module MO (the term onthe right side of Expression (7) shown above) and the rate of increasechange in the gear strength in accordance with the decrease in thetransmission load in response to the increase in the pitch circlediameter PD (the term on the right side of Expression (8) shown above).

Gear Strength Change Rate in Accordance with Increase in Pitch ConeDistance=(PCD2/PCD1)³  (9)

Expression (9) is represented by a line L6 in FIG. 9. From the line L6,it is learned that as the pitch cone distance PCD becomes larger, thegear strength is increased greatly.

With these taken into consideration, the combination of thenumber-of-teeth ratio Z1/Z2 and the pitch cone distance PCD isdetermined such that: the decrease in the gear strength based onApproach [1] given above (the increase in the number-of-teeth ratio) issufficiently compensated for by the increase in the gear strength basedon Approach [2] given above (the increase in the pitch cone distance) soas to make the overall gear strength of the differential device equal toor greater than the gear strength of the conventional existingdifferential device.

For example, 100% of the gear strength of the side gear S of thereference differential device D′ can be kept by setting the gearstrength change rate in accordance with the increase in thenumber-of-teeth ratio (i.e., the term on the right side of Expression(6) given above) obtained based on Approach [1] given above and the gearstrength change rate in accordance with the increase in the pitch conedistance (i.e., the term on the right side of Expression (9) givenabove) obtained based on Approach [2] given above, such that themultiplication of these gear strength change rates becomes equal to100%. Thereby, the relationship between the number-of-teeth ratio Z1/Z2and the rate of change in the pitch cone distance PCD for keeping 100%of the gear strength of the reference differential device D′ can beobtained from Expression (10) given below. Expression (10) isrepresented by a line L7 in FIG. 10 when the number Z2 of teeth of thepinion P is 10.

$\begin{matrix}{\begin{matrix}{{{PCD}\; {2/{PCD}}\; 1} = \left( {100{\%/\begin{matrix}{{Gear}{\; \; \;}{Strength}\mspace{14mu} {Change}} \\{{Rate}\mspace{14mu} {in}\mspace{14mu} {Accordance}\mspace{14mu} {with}} \\{{Number}\text{-}{of}\text{-}{Teeth}\mspace{14mu} {Ratio}}\end{matrix}}} \right)^{\frac{1}{3}}} \\{= \left\{ \frac{\frac{1}{196 \cdot {\sin^{3}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}{Z\; {1^{2} \cdot {\sin^{3}\left( {\tan^{- 1}\frac{7}{5}} \right)}}} \right\}^{\frac{1}{3}}} \\{= {\left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin \left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin \left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}\end{matrix}\quad} & (10)\end{matrix}$

Like this, Expression (10) represents the relationship between thenumber-of-teeth ratio Z1/Z2 and the rate of change in the pitch conedistance PCD for keeping 100% of the gear strength of the referencedifferential device D′ when the number-of-teeth ratio Z1/Z2 is equal to14/10 (see FIG. 10). The rate of change in the pitch cone distance PCDrepresented by the vertical axis in FIG. 10 can be converted into aratio of d2/PCD where d2 denotes a shaft diameter of the pinion shaft PS(i.e., the pinion support portion) supporting the pinion P.

TABLE 1 SHAFT PCD DIAMETER (d2) d2/PCD 31 13 42% 35 15 43% 38 17 45% 3917 44% 41 18 44% 45 18 40%

To put it concretely, in the conventional existing differential device,the increase change in the pitch cone distance PCD correlates with theincrease change in the shaft diameter d2 as shown in Table 1, and can berepresented by a decrease in the ratio of d2/PCD when d2 is constant. Inaddition, in the conventional existing differential device, d2/PCD fallswithin a range of 40% to 45% as shown in Table 1 given above when theconventional existing differential device is the reference differentialdevice D′, and the gear strength increases as the pitch cone distancePCD increases. Judging from these, the gear strength of the differentialdevice can be made equal to or greater than the gear strength of theconventional existing differential device by determining the shaftdiameter d2 of the pinion shaft PS and the pitch cone distance PCD suchthat at least d2/PCD is equal to or less than 45%, when the differentialdevice is the reference differential device D′. In other words, when thedifferential device is the reference differential device D′, it sufficesif d2/PCD≦0.45 is satisfied. In this case, when PCD2 denotes the pitchcone distance PCD which is changed to become larger or less than thepitch cone distance PCD1 of the reference differential device D′, itsuffices if

d2/PCD2≦0.45/(PCD2/PCD1)  (11)

is satisfied. Furthermore, the application of Expression (11) toExpression (10) given above can convert the relationship between d2/PCDand the number-of-teeth ratio Z1/Z2 into Expression (12) given below.

$\begin{matrix}{\begin{matrix}{{d\; {2/{PCD}}} \leqq {0.45/\left( {{PCD}\; {2/{PCD}}\; 1} \right)}} \\{= {0.45/\left\{ {\left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin \left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin \left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}} \right\}}} \\{= {0.45 \cdot \left( \frac{14}{Z\; 1} \right)^{\frac{2}{3}} \cdot \frac{\sin \left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}{\sin \left( {\tan^{- 1}\frac{7}{5}} \right)}}}\end{matrix}\quad} & (12)\end{matrix}$

When the Expression (12) is equal, Expression (12) can be represented bya line L8 in FIG. 11 if the number Z2 of teeth of the pinion P is 10.When the Expression (12) is equal, the relationship between d2/PCD andthe number-of-teeth ratio Z1/Z2 keeps 100% of the gear strength of thereference differential device D′.

Meanwhile, in conventional existing differential devices, usually, notonly the number-of-teeth ratio Z1/Z2 equal to 1.4 used above to explainthe reference differential device D′ but also the number-of-teeth ratioZ1/Z2 equal to 1.6 or 1.44 is adopted. This needs to be taken intoconsideration. Based on the assumption that the reference differentialdevice D′ (Z1/Z2=1.4) guarantee the necessary and sufficient gearstrength, that is, 100% of gear strength, it is learned, as being clearfrom FIG. 8, that the gear strength of conventional existingdifferential devices in which the number-of-teeth ratio Z1/Z2 is 16/10is as low as 87% of the gear strength of the reference differentialdevice D′. The general practice, however, is that the gear strength lowat that level is accepted as practical strength and actually used forconventional existing differential devices. Judging from this, one mayconsider that gear strength which needs to be sufficiently secured forand is acceptable for the differential device which is thinned in theaxial direction is at least equal to, or greater than, 87% of the gearstrength of the reference differential device D′.

From the above viewpoint, first, a relationship for keeping 87% of thegear strength of the reference differential device D′ is obtainedbetween the number-of-teeth ratio Z1/Z2 and the rate of change in thepitch cone distance PCD. The relationship can be expressed withExpression (10′) given below by performing a calculation by emulatingthe process of deriving Expression (10) given above (i.e., a calculationsuch that the multiplication of the gear strength change rate inaccordance with the increase in the number-of-teeth ratio (i.e., theterm on the right side of Expression (6) given above) and the gearstrength change rate in accordance with the increase in the pitch conedistance (i.e., the term on the right side of Expression (9) givenabove) becomes equal to 87%).

$\begin{matrix}{\begin{matrix}{{{PCD}\; {2/{PCD}}\; 1} = \left( {87{\%/\begin{matrix}{{Gear}{\; \; \;}{Strength}\mspace{14mu} {Change}} \\{{Rate}\mspace{14mu} {in}\mspace{14mu} {Accordance}\mspace{14mu} {with}} \\{{Number}\text{-}{of}\text{-}{Teeth}\mspace{14mu} {Ratio}}\end{matrix}}} \right)^{\frac{1}{3}}} \\{= \left\{ \frac{\frac{0.87}{196 \cdot {\sin^{3}\left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}{Z\; {1^{2} \cdot {\sin^{3}\left( {\tan^{- 1}\frac{7}{5}} \right)}}} \right\}^{\frac{1}{3}}} \\{= {{0.87^{\frac{1}{3}} \cdot \left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}}}\frac{\sin \left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin \left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}\end{matrix}\quad} & \left( 10^{\prime} \right)\end{matrix}$

Thereafter, when Expression (11) given above is applied to Expression(10′) given above, the relationship between d2/PCD and thenumber-of-teeth ratio Z1/Z2 for keeping 87% or more of the gear strengthof the reference differential device D′ can be converted into Expression(13) given below. However, the calculation is performed using thefollowing rules that: the number of significant figures is three for allthe factors, except for factors expressed with variables; digits belowthe third significant figure are rounded down; and although the resultof the calculation cannot avoid approximation by an calculation error,the mathematical expression uses the equals sign because the error isnegligible.

$\begin{matrix}{\begin{matrix}{{d\; {2/{PCD}}} \leqq {0.45/\left\{ {0.87^{\frac{1}{3}} \cdot \left( \frac{Z\; 1}{14} \right)^{\frac{2}{3}} \cdot \frac{\sin \left( {\tan^{- 1}\frac{7}{5}} \right)}{\sin \left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}} \right\}}} \\{= {{3.36/\left( \frac{1}{Z\; 1} \right)^{\frac{2}{3}}} \cdot {\sin \left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}}\end{matrix}{\quad\quad}} & (13)\end{matrix}$

When the Expression (13) is equal, Expression (13) can be represented byFIG. 11 (more specifically, by a line L9 in FIG. 11) if the number Z2 ofteeth of the pinion P is 10. In this case, an area corresponding toExpression (13) is an area on and under the line L9 in FIG. 11. Inaddition, a specific area (a hatched area in FIG. 11) satisfyingExpression (13) and located on the right side of a line L10 in FIG. 11where the number-of-teeth ratio Z1/Z2>2.0 is satisfied is an area forsetting Z1/Z2 and d2/PCD which enable at least 87% or more of the gearstrength of the reference differential device D′ to be securedparticularly for the differential device thinned in the axial directionwhere the number Z2 of teeth of the pinion P is 10 and thenumber-of-teeth ratio Z1/Z2 is greater than 2.0. For reference, a blackdiamond in FIG. 11 represents an example where the number-of-teeth ratioZ1/Z2 and d2/PCD are set at 40/10 and 20.00%, respectively, and a blacktriangle in FIG. 11 represents an example where the number-of-teethratio Z1/Z2 and d2/PCD are set at 58/10 and 16.67%, respectively. Theseexamples fall within the specific area. A result of a simulation forstrength analysis on these examples has confirmed that the gear strengthequal to or greater than those of the conventional differential devices(more specifically, the gear strength equal to or greater than 87% ofthe gear strength of the reference differential device D′) wereobtained.

Thus, the thinned differential device falling within the specific areais configured as the differential device which, as a whole, issufficiently reduced in width in the axial direction of the outputshafts while securing the gear strength (for example, static torsionload strength) and the maximum amount of torque transmission atapproximately the same levels as the conventional existing differentialdevices which are not thinned in the axial direction thereof.Accordingly, it is possible to achieve effects of: being capable ofeasily incorporating the differential device in a transmission system,which is under many layout restrictions around the differential device,with great freedom and no specific difficulties; being extremelyadvantageous in reducing the size of the transmission system; and thelike.

Moreover, when the thinned differential device in the specific area has,for example, the structure of the above-mentioned embodiment (morespecifically, the structures shown in FIGS. 1 to 6), the thinneddifferential device in the specific area can obtain an effect derivedfrom the structure shown in the embodiment.

It should be noted that although the foregoing descriptions (thedescriptions in connection with FIGS. 8, 10, 11 in particular) have beenprovided for the differential device in which the number Z2 of teeth ofthe pinion P is set at 10, the present invention is not limited to this.For example, when the number Z2 of teeth of the pinion P is set at 6, 12and 20, too, the thinned differential device capable of achieving theabove effects can be represented by Expression (13), as shown by hatchedareas in FIGS. 12, 13 and 14. In other words, Expression (13) derived inthe above-described manner is applicable regardless of the change in thenumber Z2 of teeth of the pinion P. For example, even when the number Z2of teeth of the pinion P is set at 6, 12 and 20, the above effects canbe obtained by setting the number Z1 of teeth of the side gear S, thenumber Z2 of teeth of the pinion P, the shaft diameter d2 of the pinionshaft PS and the pitch cone distance PCD such that Expression (13) issatisfied, like in the case where the number Z2 of teeth of the pinion Pis set at 10.

Furthermore, for reference, a black diamond in FIG. 13 represents anexample where when the number Z2 of teeth of the pinion P is 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 48/12 and 20.00%,respectively, and a black triangle in FIG. 13 represents an examplewhere when the number Z2 of teeth of the pinion P is 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 70/12 and 16.67%,respectively. A result of a simulation for strength analysis on theseexamples has confirmed that the gear strength equal to or greater thanthose of the conventional differential devices (more specifically, thegear strength equal to or greater than 87% of the gear strength of thereference differential device D′) were obtained. Moreover, theseexamples fall within the specific area, as shown in FIG. 13.

As comparative examples, let us show examples which do not fall withinthe specific area. A white star in FIG. 11 represents an example wherewhen the number Z2 of teeth of the pinion P is for example 10, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 58/10 and 27.50%,respectively, and a white circle in FIG. 11 represents an example wherewhen the number Z2 of teeth of the pinion P is for example 10, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 40/10 and 34.29%,respectively. A white star in FIG. 13 represents an example where whenthe number Z2 of teeth of the pinion P is for example 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 70/12 and 27.50%,respectively, and a white circle in FIG. 13 represents an example wherewhen the number Z2 of teeth of the pinion P is for example 12, thenumber-of-teeth ratio Z1/Z2 and d2/PCD are set at 48/12 and 34.29%,respectively. A result of a simulation for strength analysis on theseexamples has confirmed that the gear strength equal to or greater thanthose of the conventional differential devices (more specifically, thegear strength equal to or greater than 87% of the gear strength of thereference differential device D′) were not obtained. In other words, theabove effects cannot be obtained from the examples which do not fallwithin the specific area.

Although the embodiments of the present invention have been described,the present invention is not limited to the foregoing embodiments.Various design changes may be made to the present invention within ascope not departing from the gist of the present invention.

For example, the foregoing embodiments have shown the differentialdevice in which: the speed reduction gear mechanism RG formed from theplanetary gear mechanism is adjacently placed on the one side of thedifferential case DC as the input member; the output-side element(carrier 23) of the speed reduction gear mechanism RG is connected tothe differential case DC (cover portion C′); and the power from thepower source is transmitted to the differential case DC via the speedreduction gear mechanism RG. However, an output-side element of a speedreduction gear mechanism formed from a gear mechanism other than theplanetary gear mechanism may be connected to the differential case DC.

Furthermore, without using the above mentioned speed reduction gearmechanism, an input tooth portion (final driven gear, final gear)receiving the power from the power source may be integrally formed on,or afterward fixed to, the outer peripheral portion of the differentialcase DC so that the power from the power source is transmitted to thedifferential case DC via the input tooth portion.

Moreover, the foregoing embodiments have been shown in which thelubricant oil existing around the outer ends of the boss portions Cb ofthe cover portions C, C′ in the transmission case M is capable of beingfed to the oil reserving portions T on the inner end sides of the bossportions Cb, and accordingly to the oil grooves G, using the recessedgrooves 8, 8′ in the inner peripheries of the boss portions Cb of thecover portions C, C′. Nevertheless, instead of, or in addition to, therecessed grooves 8, 8′ like this, oil supply passages guiding thesplashed lubricant oil inside the transmission case M to the oilreserving portions T or the inner end portions of the oil grooves G maybe provided in appropriate places in the differential case DC (forexample, the side wall portions Cs and the boss portions Cb).Incidentally, in that case, the splashed lubricant oil inside thetransmission case M may naturally flow into the oil supply passages, ormay be actively supplied to the oil supply passages using anunillustrated oil pump.

Besides, in the foregoing embodiments, the washers W are set such thatthe inner end portions in the radial direction of the washers W arelocated further radially outward than the inner ends in the radialdirection of the back surface portions fg of the tooth portions Sg ofthe side gears S. Nevertheless, the present invention is not limited tothis. For example, the inner end portions in the radial direction of thewashers W may extend to the same position as the inner ends in theradial direction of the back surface portions fg of the tooth portionsSg of the side gears S. Thereby, it is possible to more effectivelyinhibit the decrease in the support rigidity for the back surfaceportions fg of the tooth portions Sg of the side gears S where largeload burden is applied.

In addition, the foregoing embodiments have been shown in which the backsurfaces of the pair of the side gears S are respectively covered withthe pair of dedicated cover portions C, C′ of the differential case DC.In the present invention, however, the dedicated cover portion may beprovided to only the back surface of one of the side gears S. In thatcase, for example, a driving member (for example, the carrier 23 of thespeed reduction gear mechanism RG) to be located upstream of a powertransmission passage may be arranged on a side with no dedicated coverportion, of the differential case DC, so that the driving member and thedifferential case DC are connected to each other. In that case, thedriving member concurrently serves as the cover portion C′ so that thedriving member and the differential case DC form the input member of thepresent invention.

In addition, although the foregoing embodiments have been shown in whichthe differential device D allows the difference in rotational speedbetween the left and right axles, the differential device of the presentinvention may be carried out as a center differential configured toabsorb the difference in rotational speed between front wheels and rearwheels.

What is claimed is:
 1. A differential device comprising: an input memberinputted with driving force; a differential gear supported in the inputmember and being able to rotate relative to the input member and revolvearound a rotation center of the input member in accordance with arotation of the input member; a pair of output gears each including atooth portion placed in mesh with the differential gear, and a shaftportion located radially inward of the tooth portion; a washerinterposed between the input member and a back surface of the toothportion of each of the output gears; and an oil groove provided in arecess shape in a surface of the input member facing a back surface ofeach of the output gears, the oil groove extending from a periphery ofthe shaft portion of the output gear to a back surface of the washer,wherein the oil groove is arranged offset in a peripheral direction ofthe output gear from a meshing portion between the tooth portion and thedifferential gear.
 2. The differential device according to claim 1,wherein the input member includes a side wall portion facing the backsurface of each of the output gears, the side wall portion includes aplurality of through-holes or recessed holes arranged at intervals in aperipheral direction of the input member, and the oil groove is arrangedpassing through between two of the through-holes or recessed holesplaced adjacent to each other in the peripheral direction.
 3. Thedifferential device according to claim 1, wherein an oil reservingportion is provided in a recess shape in an inner peripheral portion ofa surface of the input member facing each of the output gears, the oilreserving portion facing an outer periphery of the shaft portion of theoutput gear.
 4. The differential device according to claim 2, wherein anoil reserving portion is provided in a recess shape in an innerperipheral portion of a surface of the input member facing each of theoutput gears, the oil reserving portion facing an outer periphery of theshaft portion of the output gear.
 5. The differential device accordingto claim 1, wherein the oil groove is arranged near the meshing portionin the peripheral direction of the output gear.
 6. The differentialdevice according to claim 2, wherein the oil groove is arranged near themeshing portion in the peripheral direction of the output gear.
 7. Thedifferential device according to claim 3, wherein the oil groove isarranged near the meshing portion in the peripheral direction of theoutput gear.
 8. The differential device according to claim 4, whereinthe oil groove is arranged near the meshing portion in the peripheraldirection of the output gear.
 9. The differential device according toclaim 1, wherein as seen in a projection plane orthogonal to a rotationaxis of the output gears, a pair of the oil grooves are arranged withthe meshing portions interposed therebetween.
 10. The differentialdevice according to claim 2, wherein as seen in a projection planeorthogonal to a rotation axis of the output gears, a pair of the oilgrooves are arranged with the meshing portions interposed therebetween.11. The differential device according to claim 3, wherein as seen in aprojection plane orthogonal to a rotation axis of the output gears, apair of the oil grooves are arranged with the meshing portionsinterposed therebetween.
 12. The differential device according to claim4, wherein as seen in a projection plane orthogonal to a rotation axisof the output gears, a pair of the oil grooves are arranged with themeshing portions interposed therebetween.
 13. The differential deviceaccording to claim 5, wherein as seen in a projection plane orthogonalto a rotation axis of the output gears, a pair of the oil grooves arearranged with the meshing portions interposed therebetween.
 14. Thedifferential device according to claim 6, wherein as seen in aprojection plane orthogonal to a rotation axis of the output gears, apair of the oil grooves are arranged with the meshing portionsinterposed therebetween.
 15. The differential device according to claim7, wherein as seen in a projection plane orthogonal to a rotation axisof the output gears, a pair of the oil grooves are arranged with themeshing portions interposed therebetween.
 16. The differential deviceaccording to claim 8, wherein as seen in a projection plane orthogonalto a rotation axis of the output gears, a pair of the oil grooves arearranged with the meshing portions interposed therebetween.
 17. Adifferential device according to claim 1, wherein the differential gearis supported in the input member via a differential gear support portionsupported in the input member, and${d\; {2/{PCD}}} \leqq {3.36 \cdot \left( \frac{1}{Z\; 1} \right)^{\frac{2}{3}} \cdot {\sin \left( {\tan^{- 1}\frac{Z\; 1}{Z\; 2}} \right)}}$is satisfied, and Z1/Z2>2 is satisfied, where Z1, Z2, d2 and PCD denotethe number of teeth of each of the output gears, the number of teeth ofthe differential gear, a diameter of the differential gear supportportion and a pitch cone distance, respectively.
 18. The differentialdevice according to claim 17, wherein Z1/Z2≧4 is satisfied.
 19. Thedifferential device according to claim 17, wherein Z1/Z2≧5.8 issatisfied.